Move deleted messages to the right trash

Accounts that do not have a folder simply called 'Trash' [*] were unable to delete messages. This fix uses the special use flags to find out wich folder is the actual trash.

[*] GMail's trash for example is "[Gmail]/Trash"

Because a mailbox doesn't know about other mailboxes, the deleteMessage() method was moved up to the Account class.

:warning:  Please don't test on your real/main/production mail account first :) ALthough at worst nothing should be really lost, since this only *moves* messages to the trash...